Java 什么是premain()以及如何调用它?
我从未听说过Java 什么是premain()以及如何调用它?,java,methods,Java,Methods,我从未听说过premain,我觉得这样问有点愚蠢,但建议运行它以获得Instrumentation对象 但是如何调用该函数,或者如何使其被调用 package playground; import java.lang.instrument.Instrumentation; public class ObjectSizeFetcher { private static Instrumentation instrumentation; public static void pre
premain
,我觉得这样问有点愚蠢,但建议运行它以获得Instrumentation
对象
但是如何调用该函数,或者如何使其被调用
package playground;
import java.lang.instrument.Instrumentation;
public class ObjectSizeFetcher {
private static Instrumentation instrumentation;
public static void premain(String args, Instrumentation inst) {
instrumentation = inst;
}
public static long getObjectSize(Object o) {
return instrumentation.getObjectSize(o);
}
}
premain
是与java.lang.instrument
包关联的机制,用于加载在java程序中更改字节码的“代理”
该机制在中进行了解释
其要点是“代理”部署在一个jar中,jar的清单中有一个特殊的条目,它告诉检测包在哪里查找premain方法。您引用的源应该是一个简单代理。最小可运行示例 GitHub上游: META-INF/MANIFEST.MF
Premain-Class: Sizeof
Sizeof.java
import java.lang.instrument.Instrumentation;
final public class Sizeof {
private static Instrumentation instrumentation;
public static void premain(String args, Instrumentation inst) {
instrumentation = inst;
}
public static long sizeof(Object o) {
return instrumentation.getObjectSize(o);
}
}
Main.java
final public class Main {
public static void main(String [] args) {
System.out.println("Object");
System.out.println(Sizeof.sizeof(new Object()));
System.out.println("/\"\"");
System.out.println(Sizeof.sizeof(""));
System.out.println("/\"abc\"");
System.out.println(Sizeof.sizeof("abc"));
System.out.println("int[0]");
System.out.println(Sizeof.sizeof(new int[0]));
System.out.println("int[10]");
System.out.println(Sizeof.sizeof(new int[10]));
class OneInt {
public int i;
}
System.out.println("OneInt");
System.out.println(Sizeof.sizeof(new OneInt()));
class TwoInts {
public int i;
public int j;
}
System.out.println("TwoInts");
System.out.println(Sizeof.sizeof(new TwoInts()));
class IntArray0 {
int[] i = new int[0];
}
System.out.println("IntArray0");
System.out.println(Sizeof.sizeof(new IntArray0()));
class IntArray10 {
int[] i = new int[10];
}
System.out.println("IntArray10");
System.out.println(Sizeof.sizeof(new IntArray10()));
}
}
生成文件
all:
javac *.java
jar -cfm Sizeof.jar META-INF/MANIFEST.MF Sizeof.class
java -ea -javaagent:Sizeof.jar Main
样本输出:
Object
16
/""
24
/"abc"
24
int[0]
16
int[10]
56
OneInt
16
TwoInts
24
IntArray0
16
IntArray10
16
在Ubuntu 16.10、Java HotSpot 1.8.0_92中进行测试。您的问题的答案是关于以下源代码的: